A23658. 下面C++代码用于交换两个正整数a和b的值，不能实现交换的代码是（ ）。

cout << "输入第一个正整数: ";
cin >> a;
cout <<"输入第二个正整数: ";
cin >> b;
temp = a;
a = b;
b = temp;
cout << "a=" << a << " b=" << b << endl;

cout << "输入第一个正整数: ";
cin >> a;
cout <<"输入第二个正整数: ";
cin >> b;
b = a - b;
a = a - b;
b = a + b;
cout << "a=" << a << " b=" << b << endl;

cout << "输入第一个正整数: ";
cin >> a;
cout <<"输入第二个正整数: ";
cin >> b;
a = a + b;
b = a - b;
a = a - b;
cout << "a=" << a << " b=" << b << endl;

cout << "输入第一个正整数: ";
cin >> a;
cout <<"输入第二个正整数: ";
cin >> b;
a, b = b, a;
cout << "a=" << a << " b=" << b << endl;